I am not sure about the sleep with Windows. My MacBook Pro hibernates and takes about 20-30 seconds to start up while running Windows XP.
Officially the GPS antenna does require a clear view of the sky. It usually works when covered by piece of plastic, glass etc. I am not sure where exactly you want to mount it, keep in mind that if you put it under the dashboard, you are also reducing its view of the sky by surrounding it with all the metal in the car, not just the plastic cover above.
I do not have experience with Sirius, but XM satelite radio is even more sensitive to good sky view than GPS.